In general, a servo system comprises a speed control loop for generating a torque command corresponding to the difference between a command speed from a position control loop and an actual motor speed. The speed control loop of a typical servo system shown in FIG. 1 is arranged to effect a proportional-plus-integral control or integral-plus-proportional control when a parameter P assumes a value of "1" or "0", respectively. In FIG. 1, symbols Vc and Tc respectively denote a speed command signal and an actual speed signal; k1 and k2, an integral gain and a proportional gain; kt, denotes a torque constant; and Jm, rotor inertia.
The speed control loop in FIG. 1 is a secondary control system and the response characteristic thereof is determined in dependence on the speed loop gain. That is, when the loop gain is excessively small, the motor rotation will not immediately follow each of movement command pulses supplied from the servo system and a control lag occurs in such a way that the motor cannot start rotating before a plurality of pulses are accumulated, thus causing a low-frequency undulation in the motor speed. Further, the servo system is liable to be affected by disturbance. On the other hand, when the loop gain is excessively large, the control stability is degraded. If the loop gain is further increased, a mechanical system drivingly coupled to the servomotor resonates and causes an oscillation in the servo system. In this case, in machine tools, for example, vibration occurs even at the time of cutting and feeding operation at a normal motor speed.
Therefore, it is necessary to appropriately set the loop gain, i.e., the integral gain k1 and the proportional gain k2, and more specifically, both the gains k1 and k2 must be set to a large value falling within a range in which no oscillation occurs in the servo system. To this end, in view of the fact that the gain k1 varies as a function of a cut-off frequency fn and the gain k2 varies as a function of the cut-off frequency fn and a damping factor (attenuation factor) .xi., as indicated in the following equations (1) and (2), conventionally, the parameters fn and .xi. for each machine equipped with the servo system are set in a trial and error manner, for appropriate determination of the loop gain. EQU k1=(Jm/Kt).multidot.(2.pi.fn).sup.2 ( 1) EQU k2=(Jm/Kt).multidot.2.xi..multidot.2.pi.fn (2)
Therefore, it requires much effort and a long time to determine the loop gain.